chore: update documentation links for 0.17.0 release

[andygrove]

Which issue does this PR close?

N/A. This is a routine documentation update following the 0.17.0 release.

Rationale for this change

Now that Comet 0.17.0 has been published, the documentation site needs to reference 0.17.x as the current stable release, move 0.16.x into the list of older versions, and publish the 0.17.0 changelog.

What changes are included in this PR?

Documentation links:

• docs/generate-versions.py: set latest_released_version to 0.17.0 and slide the previous_versions window to ["0.14.0", "0.15.0", "0.16.0"]
• docs/build.sh: update the cleanup of locally cloned release branches to comet-0.14, comet-0.15, comet-0.16
• docs/source/user-guide/index.md: present 0.17.x as the current stable release
• docs/source/user-guide/older-versions.md: add 0.16.x and drop 0.13.x from the older versions list, and point the stable-release link at 0.17

This keeps the published guides at the current release plus the three prior versions, matching the convention used for previous releases.

Changelog:

• docs/source/changelog/0.17.0.md: add the 0.17.0 changelog
• docs/source/changelog/index.md: register 0.17.0 in the changelog index

How are these changes tested?

Documentation-only change. Verified by building the docs locally per docs/README.md.

[coderfender]

Thank you @andygrove